MEPS 245:69-82 (2002)  -  doi:10.3354/meps245069

ABSTRACT: The large agglutinated foraminiferan Bathysiphon capillare de Folin (Protista) was an important component of the macrofauna in box core samples recovered at a 950 m site on the southern flank of the Wyville-Thomson Ridge, northern Rockall Trough. The long, narrow, very smooth, flexible tubes of B. capillare reached a maximum length of almost 10 cm. Densities ranged from 100 to 172 ind. m^-2, a figure that represents at least 5 to 9% of metazoan macrofaunal densities. This infaunal species usually adopted a more or less horizontal orientation within the upper 5 cm layer of brownish sandy silt. Its cytoplasm yielded a diverse spectrum of fatty acids. These included various monounsaturated fatty acids (39% of total), mainly 18:1(n-7), 20:1(n-9) and 22:1(n-7), the polyunsaturated fatty acids (PUFA) 20:4(n-6), 20.5(n-3) and 22.6(n-3), and non-methylene diene-interrupted fatty acids (NMIDS), particularly 22:27,13 and 22:27,15. The spectrum of PUFAs is consistent with the ingestion by B. capillare of phytodetrital material. However, the presence of NMIDS, reported here for the first time in a protist, provides evidence for a substantial bacterial component in the diet. Viewed by scanning electron microscopy (SEM), the cytoplasm occupied a narrow space between the inner organic test lining and an inner core of detritus around which it formed a sheath. The core material included numerous spherical structures (2 to 10 µm diameter) that we interpret as stercomata (waste pellets composed largely of mineral grains). Taken together, our observations suggest that B. capillare is a deposit feeder, consuming sediment, detritus and associated bacteria. Other deep infaunal foraminiferans probably have a similar diet, leading us to speculate that these protists, which are often abundant in dysoxic settings, may contribute significantly to carbon cycling in the deep sea. Since B. capillare and similar large Bathysiphon species can be recovered in good condition from bathyal depths, they may prove to be ideal experimental subjects for addressing some fundamental issues in the lipid biochemistry of deep-sea benthic organisms.

KEY WORDS: Deep sea · Diets · Fatty acids · Foraminiferans · Lipids · Macrofauna · NMIDS · PUFA